1. Field of the Invention
The present invention relates to a light valve image projection apparatus which projects an optical image formed on a light valve onto a projection screen in an expanded projection manner.
2. Description of the Prior Art
In order to obtain a large screen image, there has been conventionally known a method of forming an optical image on a light valve according to a video and irradiating light beams to the optical image formed on the light valve to project the optical image onto a projection screen by means of a projection lens system an expanded projection manner. Lately, a growing attention has been paid to a light valve image projection apparatus in which a liquid crystal display unit (referred to as "LCD" hereinafter) is employed as a light valve.
FIG. 8 shows a schematic form of a light valve image projection apparatus employing an LCD as a light valve. Representative light bundles of rays A, B and C emitted from a light source 1 pass through an LCD 2 to be applied as incident light rays to a projection lens assembly 3. The LCD 2 comprises of an input side polarizing plate 4, a liquid crystal cell layer 5 and an output side polarizing plate 6. The liquid crystal cell layer 5 is formed by sealing twisted nematic liquid crystals 9 between two glass plates 7 and 8. Each of the glass plates 7 and 8 is provided with transparent pixel electrodes arranged in a matrix on the inner surface thereof in contact with the liquid crystals 9.
The polarization axis of the input side polarizing plate 4 and the polarization axis of the output side polarizing plate 6 cross each other at right angles with each polarization axis being inclined at an angle of 45.degree. with respect to the vertical scanning direction of the liquid crystal cell layer 5. When no voltage is applied to the transparent pixel electrodes arranged in the glass plates 7 and 8, a linearly polarized light ray output through the input side polarizing plate 4 is rotated at an angle of 90.degree. due to an optical rotatory power of the liquid crystal cell layer 5 to achieve a maximum transmittance of the polarized light rays passing through the LCD 2. When applying a voltage to the transparent pixel electrodes, the effect of the optical rotatory power in the liquid crystal cell layer 5 is reduced as the applied voltage changes to thereby reduce the transmittance of the polarized light rays.
Thus, an optical image corresponding to a video signal is formed in the LCD 2 as a variance of the transmittance of the polarized light rays, and the obtained optical image of the video signal is projected onto a projection screen 10 in an expanded projection manner by means of the projection lens assembly 3.
An LCD employing twisted nematic liquid crystals exhibits a highest contrast ratio when a voltage is applied to the LCD in such a manner that the alignment direction of the liquid crystal molecules coincides with a travelling direction of an incident light ray.
FIG. 9 shows an example of the relationship between an incident angle of an incident light ray and an contrast ratio of an LCD. A principal light ray which is the dominant ray of a conical bundle of rays incident on the optical image plane of the liquid crystal cell layer is directed within a plane including the normal of the liquid crystal cell layer 5 and the vertical scanning direction of the LCD. It is now assumed that the principal ray and the normal of the liquid crystal cell layer cross each other at an angle .theta..
As is apparent from FIG. 9, a highest contrast ratio is achieved when the angle .theta. is .theta..sub.0 slightly inclined from an angle 0.degree., and the contrast ratio decreases as the angle .theta. deviates from the angle .theta..sub.0. It is noted here that the angle .theta. is defined as "the angle of the principal ray" hereinafter.
In the case of an arrangement as shown in FIG. 8, generally the angle .theta. of the principal ray incident on the liquid crystal cell layer 5 within an effective field angle increases toward the periphery from the optical axis (.theta.=0.degree.) of the projection lens 3.
As shown in FIG. 10, in the above case, the contrast ratio in the vertical direction of the projected image on the projection screen takes its maximum value when the angle .theta.=.theta..sub.0 and the contrast ratio decreases as the angle deviates from the contrast peak angle .theta..sub.0. Therefore, the contrast ratio has its peak at a place displaced from the center of the projected image and is lower at places above and below the peak, which incurs asymmetric nonuniformity of contrast in the projected image thereby adversely influencing the picture quality.
In order to eliminate the above-mentioned problems and improve the uniformity of contrast, Sakamoto et al. discloses a projection apparatus having a pair of convex lenses on both sides of a liquid crystal panel in Japanese Patent Application Laid-Open No. HEI-3-146941, while Nakamura discloses a reflector and a light ray guide in combination in an illumination optical system in the Japanese Patent Application Laid-Open No. HEI-4-22938, parallel light rays being incident on the liquid crystal panel in each of the disclosed apparatus. However, in either one of the conventional apparatus, the angle of the principal ray incident on the liquid crystal panel is 0.degree. but not equal to .theta..sub.0, which is consequently insufficient for obtaining a high contrast ratio.
Various apparatus in which a liquid crystal panel is inclined at a specified angle toward the maximum contrast view angle from the vertical plane with respect to the optical axis are disclosed in Japanese Patent Application Laid-Open No. SHO-62-186225 by Nakamura et al., in U.S. Pat. No. 4,824,210 by Shimazaki, in Japanese Patent Application Laid-Open NO. HEI-3-71110 by Kishimoto et al., and in Japanese Patent Application Laid-Open No. HEI-3-75617 by Hosoi et al. It is also possible to make the angle of the principal ray incident on the liquid crystal panel optimum for achieving high contrast by employing variation of the above-mentioned apparatus. However, it is necessary to incline the projection screen plane from the plane perpendicular to the optical axis of the projection lens in order to satisfactorily focus the projected image thereon. Even when the projection screen plane is inclined, a trapezoidal distortion takes place in the projected image, which makes it difficult to obtain a high quality picture.
Other apparatus employing an optical component such as a prism, a fibrous plate acting as a prism or a fine stepped prism having a sawtooth-shaped cross section disposed on the output side of the liquid crystal panel and refracting a light ray incident on the liquid crystal panel at the angle for achieving the highest contrast ratio with respect to the optical axis of the projection lens, are disclosed in Japanese Patent Application Laid-Open No. SHO-62-3227 by Tanaka et al., in U.S. Pat. No. 4,936,657 by Tejima et al., in Japanese Patent Application Laid-Open No. HEI-3-43780 by Oka et al., and in Japanese Patent Application Laid-Open No. HEI-3-259131 by Nakanishi et al. In any of the above-mentioned apparatus, the light rays incident on the liquid crystal panel have an incident angle for achieving a high contrast ratio and efficiently converge the light rays into the entrance pupil of the projection lens. However, the above-mentioned optical components are arranged between the liquid crystal panel and the projection lens, which makes it difficult to compensate for various aberrations of the projection lens. Furthermore, when using an element having a fine stepped structure, the element must be arranged in the vicinity of the optical image plane of the liquid crystal panel. Therefore, an expanded image of the fine structure fatally appears on the projected image, which also results in difficulty in obtaining a high-quality projected image in any event.
Various apparatus in which the central axes of the liquid crystal panel, projection lens, and projection screen are displaced to irradiate light rays in an oblique direction onto the liquid crystal panel for achieving the highest contrast ratio are disclosed in Japanese Patent Application Laid-Open No. SHO-63-73782 by Horiuchi et al. and in Japanese Patent Application Laid-Open No. HEI-2-5082 by Ohba.
FIG. 11 shows, schematically, the structure of the above-mentioned apparatus. The center of the image forming frame of the liquid crystal cell layer 5 is displaced vertically with respect to the optical axis of the projection lens 3 so that the field angle .theta.=.theta..sub.0, at which the projected image has the maximum contrast ratio, is subtended from a location near the center of the liquid crystal cell 5.
FIG. 12 shows the contrast ratio in the vertical direction of the projected image in the above-described apparatus. With the apparatus shown in FIG. 11, an area around the center of the projection screen can be made to exhibit the peak contrast ratio to prevent a possible occurrence of asymmetrical nonuniformity. However, the contrast ratio reduces toward the periphery of the liquid crystal cell layer, which also leaves unsolved the problem that no uniformity can be ensured throughout the area of the projected image area. Furthermore, the liquid crystal cell layer 5 is displaced in parallel in the vertical direction with respect to the optical axis of the projection lens 3, which requires the projection lens to have a wide angle and makes asymmetrical the range of field angle for use in the vertical direction of the projected image. The brightness of the projected image at an arbitrary point is proportional to the product of the fourth power of the cosine of the field angle and the vignetting factor of the projection lens, and therefore the brightness depends greatly on the field angle. Therefore, when the range of field angle for use is asymmetrical, there arises another problem that the brightness varies when the lens is in different vertical positions.